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Flawed Reasoning Question (paraphrasing):

Any announcement authorized by the head of dept. is important, however sometimes announcements are issued by people other than heads of the dept., so some announcements will inevitably turn out not to be important.

(D) assumes without warrant that just because satisfying a given condition is enough to ensure an announcement's importance, satisfying that condition is necessary for its importance.
(E) fails to distinguish between the importance of the position someone holds and the importance of what that person may actually be announcing on a particular occasion.

Why is it (D)? I picked (E) incorrectly.

Thank you for your enlightenment!

If it's from the boss, it's important. So, if it's not from the boss, sometimes it will not be important.

This is one of the cardinal mistakes on the LSAT, confusing necessary and sufficient conditions (or denying the antecedent for you old-school types).

B --> I, therefore ~B --> ~I

Essentially, this is a bad argument because it's possible that every single memo given out by the company is always important. The boss's memos are important, the VP's memos are important, the secretaries memo's are important.

Answer choice (E) suggest the argument is flawed because it doesn't distinguish between how important a person is and how important that person's message is. It's tricky, because it is true the argument doesn't do that. However, the argument doesn't need to do that, it's not making any sort of claim like that. Nothing in the stimulus suggests that people other than the head of the department are either important or unimportant.

Hope that is clear.

The reasoning is flawed because it involves what the Powerscore Bible calls "mistaken negation". Please refer to their book, the part on conditional reasoning.